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Switching high speed optical signals can require a switching matrix that is capable of switching broad bandwidth signals that can have both a very high upper cutoff frequency, i.e., 40 GHz and higher, and a very low cutoff frequency, i.e., 100 KHz or lower. In addition, the switch matrix must be able to maintain the signal fidelity over the entire frequency range. To maintain the necessary fidelity, a switch must be properly matched to minimize the mismatch reflections that will occur over the frequency range, and particularly at the higher frequencies.
Typically, isolation switches have been placed in series with the signal switching devices. The isolation switches have a low xe2x80x9conxe2x80x9d resistance and a high xe2x80x9coffxe2x80x9d resistance. The isolation switches closest to the signal switching device, on the side away from the signal propagation path are switched off to isolate the signal switching device. However, the isolation switches typically have high parasitic reactances that, although ignored at lower frequencies, adversely impact the performance of the switch matrix at high frequencies and lead to generation of amplitude, phase, and delay distortions that can cause serious deterioration in bit error rates for high speed data channels.
Therefore, it would be desirable to provide a switch matrix that provides for isolating and matching of the input signals across a very broad bandwidth.
A method and apparatus are disclosed for isolating a selected switching connection, within a switch matrix. The switching connection is isolated by forming two shunt stubs, or lumped circuit equivalents, each having an electrical length such that the input impedance of the shunt stub is high. The shunt stubs are formed on the appropriate input and output signal lines of the switch matrix on the opposite side of the input and output lines from the preferred direction for the propagation of the switched signal. It will be shown in subsequent sections of this disclosure that the use of the shunt stub architecture facilitates simplified biasing of the active isolation devices, provides for broadband isolation of the selected signal transmission path, and, together with appropriate spacing of the matrix transmission lines, minimizes the effects of the parasitic reactances of the active signal and isolation switching devices.
In particular, in one embodiment, a switch matrix is disclosed having a plurality of input and output signal lines. Each of the input signal lines has a signal input end and a non-input signal end and each of the output signal lines has a signal output end and a non-output signal end. The switching matrix switches a broadband signal of interest (having a center frequency) between a an input signal line and a signal output line using one of a plurality of signal switches. Each one of the plurality of signal switches is coupled to a single input signal line and a single output signal line. Accordingly, when a selected signal switch is activated, the signal of interest is coupled between the selected input signal line and the selected output signal line.
The switch matrix further includes a plurality of first isolation switches, each of which is associated with a single signal switch. Each of the plurality of first isolation switches are coupled to one of the plurality of output signal lines associated with the single signal switch. The isolation switch is further coupled to ground forming a tuning stub when it is activated. The length of the tuning stub is determined by the spaced apart distance each isolation switch is disposed from the single signal switch. Thus, when the first isolation switch is activated, the portion of the output line between the associated signal switch and the isolation switch forms the tuning stub of the first predetermined length. Preferably, the first predetermined length is equal to a predetermined electrical length of transmission line that is one quarter wavelength and the center frequency of the signal of interest.
The switching matrix also includes a plurality of second isolation switches, each of which is associated with a single signal switch. Each of the second plurality of isolation switches is coupled to one of the plurality of input signal lines associated with the single signal switch. Each of the plurality of isolation switches is further coupled to ground forming a tuning stub. The length of the tuning stub is determined by the spaced apart distance each isolation switch is disposed from the associated signal switch. Thus, when a second isolation switch is activated, the portion of the input line between the associated signal switch and the isolation switch forms the tuning stub of the second predetermined length. Preferably, the second predetermined length is equal to a predetermined electrical length of transmission line that is one quarter wavelength and the center frequency of the signal of interest.
In one aspect of the present invention, the plurality of signal switches and the first and second isolation switches are a plurality of semiconductor switching elements, and are selected from the group of switching diodes, thryristors, and transistors. In particular, the plurality of switching diodes are a plurality of PIN diodes, the plurality of transistors are a plurality of bipolar junction transistors, the plurality of transistors are a plurality of field effect transistors.
In another aspect of the present invention, the first and second predetermined electrical length is an odd integer multiple of the quarter-wavelength of the signal of interest. In particular, the first and second predetermined electrical distances are one-quarter wavelength of the signal of interest.
In another embodiment, the first and second stub are provided for by a plurality of lumped circuit elements that provide the same impedance transforming function as the stub. In particular, the lumped circuit is a pi-section that includes an inductor coupled to a capacitor at each end, and where each of the capacitors are further coupled to ground. Alternatively, a pi-section can be formed that includes a capacitor coupled to an inductor at each end, and where each of the inductors are further coupled to ground. The use of lumped elements facilitates size matrix reduction relative to the distributed transmission line counterpart.
Other forms, features and aspects of the above-described methods and system are described in the detailed description that follows.